A credit card is a system of payment in which money is not removed from the user's account after every transaction. The issuer lends money to the user to be paid to the merchant. The balance does not have to be paid in full each month. A credit card allows the user to revolve the balance, at the cost of having interest charged.
A line of credit is any credit facility extended to a user, such as a person or a business, by a bank or financial institution. It is like an account that can readily be tapped into if the need arises or not touched at all and saved for emergencies. A line of credit may be secured by collateral. Such a line of credit is referred to as an equity line of credit. For example, a home equity line of credit (often called HELOC) is a loan in which the lender agrees to lend a maximum amount within an agreed period (called a term), where the collateral is the user's equity in his house.
With a line of credit, the user is not advanced the entire sum up front, but uses a line of credit to borrow sums that total no more than the credit limit, similar to a credit card. Interest is only paid on the money actually taken out. The interest rate on a line of credit is typically lower than the interest rate on a credit card.